1. Field of the Invention
The present invention relates to fiber optic sensors and particularly to the multiplexing of interferometric sensors using a fiber optic ladder or star configuration of interferometric sensors and code division multiplexing to produce low crosstalk between sensors.
2. Description of the Related Art
Spread spectrum (SS) and code division multiplexed (CDM) techniques have been applied to a variety of communications applications, including optical fiber systems. This type of signal processing has also been previously investigated for optical time-domain reflectometry (OTDR) based sensing and, more recently, has been proposed and tested as a means for multiplexing interferometric sensors. In this work, the interrogating laser source is modulated using a pseudo-random bit sequence (PRBS) of length 2.sup.m -1 (maximal length sequence or m-sequence), and correlation is used to provide synchronous detection to identify specific sensor positions. A delay equal to an integer multiple of the bit (or `chip`) period separates the sensors. The received signals from the array are then encoded by delayed versions of the PRBS, and correlation techniques can be used to extract the individual signals. Although this technique may provide advantages in terms of power budget for time-division multiplexed systems, prior work in this area has been limited by excess phase noise effects arising due to mixing of time coincident pulses from different sensors and by relatively high crosstalk between sensors.